The field of the invention is that of wide passband antennas with hemispherical or quasi-hemispherical radiation patterns. More specifically, the invention relates to resonant helix antennas and especially to the power supply for said antennas.
The antenna of the invention can find application especially in mobile satellite communications between users in fixed positions and moving bodies of all kinds for example, aeronautical, maritime or land-based bodies. In this field, several satellite communication systems are being implemented or are currently being developed (these include, for example, the INMARSAT, INMARSAT-M, GLOBALSTAR, and other systems). These antennas are also valuable in the deployment of personal communications systems (PCS) using geostationary satellites.
For all these systems, which provide for links with geostationary satellites, the very great difference in incidence between the signals received or transmitted requires that the antenna should have a radiation pattern with hemispherical coverage. Furthermore, the polarisation has to be circular with a ratio of ellipticity of more than 5 dB in the useful band.
More generally, the invention can be applied in all systems requiring the use of a wide band, a radiation pattern with hemispherical coverage, circular polarisation and a good ratio of ellipticity.
In the above-mentioned fields of application, the antennas must have the above-mentioned characteristics either in a very wide passband in the range of 10% or in two neighbouring sub-bands respectively corresponding to reception and transmission.
The patent FR-89 14952 filed on behalf of the present Applicant has already described a known type of antenna particularly suited to such applications.
This antenna, called a resonant quadrifilar helix (RQH) antenna has characteristics very close to the criteria laid down in a frequency band generally limited to 5% owing to problems of impedance matching. Wider band operation is possible by using dual-layer RQH antennas. These antennas are formed by the concentric "nesting" of two electromagnetically coupled coaxial resonant quadrifilar helices.
A quadrifilar antenna is formed by four radiating strands. An exemplary embodiment is described in detail in A. Sharaiha and C. Terret, "Analysis of quadrifilar resonant helical antenna for mobile communications", IEEE--Proceedings H, Vol. 140, No. 4, August 1993.
In this structure, the radiating strands are imprinted on a thin dielectric substrate and then wound on a cylindrical medium that is radio electrically transparent. The four strands of the helix are open or short-circuited at one end and electrically connected at the other end with conductive segments positioned on the base of the lower part of the supporting cylinder. The four strands of the helix are therefore excited through these conductive segments.
This antenna conventionally requires a supply circuit that excites the different antenna strands by signals having the same amplitude in phase quadrature. There are several known techniques used to obtain a supply circuit of this kind.
In the above-mentioned document "Analysis of quadrifilar resonant helical antenna for mobile communications", this function is fulfilled by means of a structure using couplers (3 dB, -90.degree.) and a hybrid ring. This assembly is implanted on a printed circuit placed at the base of the antenna.
This technique has the advantage of being relatively simple to make and implement. By contrast, it leads to a non-negligible space requirement as compared with the antenna (which for example may have a size of about ten centimetres). This drawback makes this approach incompatible with many applications, especially when maximum miniaturisation is required.
According to a second technique described in J. L. Wong and H. E. King, "UHF satellite array nulls adjacent signals" (Microwaves & RF, March 1984), each bifilar helix may be supplied by a "folded balun" type of coaxial symmetrizer. The two bifilars helices are then excited in phase quadrature by means of a hybrid coupler.
The advantage of this method is that it requires the use of only one external hybrid element. By contrast, the symmetrizer/adapter assembly used for this type of antenna (made for example out of a coaxial section whose core and sheath form a dipole) is complex and bulky.
Furthermore, this type of assembly has the drawback of forming a sort of passband filter with a band that is still excessively narrow.
A third, more complex technique is described in C. C. Kilgus, "Resonant quadrifilar helix" (Microwave Journal, December 1970). The coaxial supply line is split at its end to form a symmetrizer. The phase quadrature is provided by adjusting the length of the strand.
This technique is used to eliminate hybrid couplers. However, it has the drawback of requiring a delicate adjustment of the length of the strand. Furthermore, the antenna is no longer symmetrical and the structure will be more complex. Besides, this method remains specifically reserved for systems using a narrow working band.